The presence of undesirable microbial contamination in food products intended for consumption is of significant concern to manufacturers, farmers, packagers, food distributors, wholesalers, retailers, consumers, and to worldwide public health. A particularly worrisome concern is bacterial contamination in packages containing food products for human consumption. The United States boasts of the safest food in the world; however, each year one in four citizens suffers from a food borne illness and some 5,000 die from something they have eaten. According to the Center for Disease Control and Prevention, each year in the United States, 76 million people contract some kind of food borne illness, 325,000 are hospitalized and 5,000 fatalities occur due to microbial contamination of consumed food. In Third World countries it has been estimated that bacterial contaminated food and water kills over two million children each year. Despite those numbers, most food borne infections are undiagnosed and unreported.
Packaging of perishable and edible food products that may be susceptible to bacterial contamination has concentrated primarily on the prevention of bacterial growth of these products as opposed to the detection of bacterial growth in these products. Hence, there is a need for an improved method for making packaging material for determining the presence or absence of contaminating microorganisms in a package.
When the packaging/wrapping element has been used for detection of microbial growth in the food stuff, such elements typically had a pH indicator entrapped in the polymer composition. See, e.g., U.S. Pat. Nos. 5,753,285 and 6,495,368 which disclose dispersion of the indicator into the polymer blend for use in food wraps. The pH indicator is sensitive to byproducts produced by microbial growth on/in the food stuff and a change in color of the indicator is taken as a measure of the extent of microbial growth. Nevertheless, such packaging elements are susceptible to leaching of the pH indicator into the food thereby contaminating the food and as such are not favored.
The use of pH indicators covalently bound to a polymeric matrix has been disclosed by, for example, Adams, et al., U.S. Pat. No. 6,391,626 wherein a functional group of the pH indicator is directly bound to a functional group of the polymeric matrix. Specifically disclosed is the covalent bonding of [2-(2,4-dinitrophenylazo)-6-(N-methyl-N-(2-hydroxysulfonyl-oxy-ethylsulfonyl)amido]-1-naphthol-3-sulfonic acid (DNSA) to polyvinyl alcohol (PVOH). It would appear that one possible covalent bond formed would be a sulfonoxy bond between the hydroxyl group of the PVOH and the sulfonic acid of DNSA.
Such direct binding of the pH indicator is limited by several factors including the presence of a suitable functionality on the pH indicator that can bind to the polymeric matrix without loss of its pH indicating properties; the stability of the resulting bond during storage and under aqueous acidic or basic conditions, and the level of indicator bound to the polymer. In the last case, Adams reports that the 500 mg of indicator is bound to 100 g of polymer (a 200:1 ratio of polymer to indicator). Further, it is recognized in the art that the use of common ester or acidamide covalent bonds between the pH indicator and the polymeric resin is less than ideal as these bonds are not stable in either acidic or alkaline environments.
Covalent binding of pH indicators through a linker group has been disclosed by Liu, et al., Journal of Polymer Science Vol. 43 1019-1027 (2006) pp. 1019-1027 and Sensors and Actuators B 107 (2005) pp. 311-315. These references disclose the formation of intermediates formed by reacting phenolphthalein, o-cresolphthalein, and phenol red with formaldehyde. These intermediates in turn are reacted with PVOH in the presence of DMSO at 100° C. to provide a defined covalent linkage. However, these references do not disclose the use of such covalently bound pH indicators for use in food storage and detection of microbial growth. In point of fact, the use of DMSO as a solvent during bonding of the indicator to the PVOH through the linker is counter-intuitive to use of the resulting polymeric film to wrap food stuffs as DMSO is known to impart a bitter taste (Merck Index, 12th Ed. P. 3309 (1996)). Accordingly, any residual DMSO in the product could contaminate the food rendering it non-sellable.